For many years wireless communication technologies have focused on providing high quality voice services to mobile stations. Recently there has been a desire to provide broadband wireless data to satisfy increasing demands for multimedia applications. Broadband wireless data consumes a large amount of radio frequency resources. Radio frequency resources are typically controlled by governmental bodies who allocate radio frequency spectrum to various wireless communication system operators and define the amount of acceptable interference to other radio frequency spectrums. Typically, a wireless communication system operator is allocated only a limited amount of radio frequency spectrum in a particular geographic area. Accordingly, this limited spectrum should be used in an efficient manner to provide wireless broadband data to as many mobile stations as possible.
In wireless communication systems radio frequency resources are allocated to different mobile stations using a scheduler. Conventional scheduling algorithms typically support Quality of Service (QoS) and attempt to maintain high throughput. In wireless communication systems these schedulers account for the location-dependent and time-varying link capacity, limited spectrum, high error-rate and user mobility. Schedulers typically take advantage of the time variation of the wireless channel when allocating resources to avoid allocating resources to a mobile station in a deep fade that causes significant packet loss, while depriving mobile stations with good channel conditions from taking advantage of the instantaneous large capacity. Therefore, conventional schedulers try to balance QoS (including differentiation between mobile stations and guaranteed QoS for particular mobile stations) and providing high network utilization.
FIG. 1 illustrates an exemplary OFDMA frame used for WiMAX communications. An OFDMA subframe includes a predetermined number of tones (which represent frequencies), and each of these tones can be reused over a predetermined number of symbols (each symbol representing a period of time). WiMAX provides 47 symbols per frame, and a varying number of tones dependent upon the total bandwidth allocated to a base station.
The downlink (DL) and uplink (UL) subframes are composed of several slots for user data. Each slot is defined by a rectangular region that comprises one or more symbols (a maximum of three in WiMAX), and a group of tones (adjacent or non-adjacent) in each symbol. Accordingly, the amount of radio frequency resources employed for transmitting information in an OFDMA system is based on the number of allocated tones and symbols. The size of these slots is fixed and known both at the transmitter and receiver. Whether a particular mobile station is scheduled in a particular frame, the size of allocated resource per frame, and the location of the allocated slot within the subframe is identified by the scheduler. The QoS parameters defined for each mobile station (or for each service flow for a mobile station), including maximum sustained data rate, maximum latency and jitter tolerated are used at the scheduler to allocate the slots.
Existing schedulers attempt to balance QoS (differentiation and guarantees) and providing high network utilization. The throughput achieved by mobile station, the packet delay, and other QoS parameters is determined based on whether the mobile station has been scheduled in a particular frame, and how many slots have been allocated to mobile station.